Process and apparatus for drying organic or inorganic materials

ABSTRACT

A method of drying organic or inorganic material or a mixture thereof, said material containing a liquid or liquids, which comprises introducing said material interiorly of a rotating rotor carrying on its periphery oscillating rotor blades, said rotor rotating about the axis of a process chamber having an inner surface with substantially axially extending ribs, said rotating rotor causing the material introduced interiorly of said rotor to be propelled radially outwardly of said axis of rotation and to expel liquid or liquids from said materials, and the oscillating rotor blades and the opposing ribs causing vibrational forces and turbulence to be induced into the outwardly flowing liquid so as to form a mist. Apparatus for carrying out said method is also described.

The invention relates to a method of drying materials, as disclosed inthe preamble of Patent Claim 1. The invention further relates to adevice for Implementation of the drying organic or inorganic materialsor mixtures thereof, said materials containing one or more liquids,wherein drying is carried out by removing said liquid or liquids asatomized droplets from the solid material and apparatus for carrying outsaid method.

Thus the invention relates to a method of removing liquids (drying) fromvarious atomized organic or Inorganic materials by transforming part ofthe liquids into mist acting as vapour or gas, thus saving evaporationheat, reducing energy consumption dramatically compared to pureevaporation of the same liquids.

In principle, all the techniques or processes of the prior art relatingto the removal of liquids from such atomized materials are based eitheron mechanical separation through cetrifugation, sedimentation etc., orsome other form of total evaporation of the liquids in the material.This invention aims to show a new way of removing liquids from thematerials described in the literature as drying, after part of theliquids may have been removed by the first-mentioned techniques.

The literature cites a number of different drying systems, in generalcomprising various ways of supplying energy to the materials with a viewto evaporation of the liquids. The reason for this need for differenttechniques has to do with the type of material, the atomization of theparticles, the type of liquid, and not least the thermodynamic values ofthe materials throughout the process.

The simplest and most trivial kind of drying is seen in the outdoordrying of washing. Here the clothes are dried under the influence of sunand wind.

Amongst others, the prior art comprises the following drying methods andprocesses:

Rotary drying, where the energy is either supplied through a flame blowninto the dryer--a so-called through-heat dryer, or by means of thermalconductance elements of various designs (pipes, plates etc.)--aso-called oil or steam dryer.

Fluidized dryer which normally consists of a vertical cylinder with acloth, or some other perforated plate in the bottom, through which isblown some kind of heated gas, heating the material and expelling theliquids through evaporation.

There is furthermore infrared dryer where the energy is delivered asinfrared radiation, different types of tunnel furnaces, and so-called"turbo-driers" where the material is dried in a vertical cylinder bymeans of spinning fans and a countercurrent of hot air or some otherheated gas.

The processes which are well known from the prior art and described indetail in a number of textbooks such as PERRY'S CHEMICAL ENGINEERINGHANDBOOK, are used for a number of drying purposes.

The problem with all the methods is that the evaporation of the liquidsrequires so much energy that the finished product would normally need tobe of a considerable commercial value, far in excess of the drying cost.

However, there are now a number of different mixtures ofmaterials/liquids where this is not the case; nevertheless it would be agreat advantage for various reasons to dry these, notably forenvironmental reasons. Such mixtures of materials/liquids might forexample include:

All types of aqueous sludge from municipal plants.

Ordinary sewage waste.

Livestock manure.

All types of filtrates from centrifuges containing liquids.

Various biological and chemical compounds from refineries, woodprocessing, and the food industry etc.

Most of these materials are characterized by being normally regarded aswaste, thus being normally viewed as not worth the costlydrying-by-evaporation process.

The objective of the present invention is to provide a process or amethod of drying such materials using less energy than current dryingprocesses, in that the liquids are not evaporated but removed throughthe transformation of a considerable portion of the said liquids intomist.

In the evaporation of a liquid, the energy requirements are expressed bythe following equation:

    Q=m·c·dt  J!

Here Q is the energy requirement in Joule, m the mass of the liquid inkg c the specific heat of the liquid in J/k °C. and dt is thetemperature difference between the evaporation temperature and theinitial temperature of the liquid.

In evaporation of water, the equation is often expressed by:

    Q=m.dh=m(h.sub.2 -h.sub.1)+k  J!

Here h₂ is the enthalpy of the steam at evaporation temperature and h₁is the enthalpy of the water at the temperature of the feed, i.e. thetemperature of the water when it is fed into the process along with thesolids, and k equals the loss in the process.

If we look at a quantity of sludge with water, the equation is expandedto:

    Q=m·x.sub.s ·c.sub.s ·dt+m·x.sub.v (h.sub.2 -h.sub.1)+k=m x.sub.s ·c.sub.s ·dt+x.sub.v ·(h.sub.2 -h.sub.1)!+k J!

Here x_(s) is the weight proportion of solids, c_(s) is specific heat ofthe solids and x_(v) is the weight proportion of the water. If there areseveral different solids in the material, the equation is expanded foreach solid ingredient with its respective weight proportion and specificheat.

Looking at a typical sludge from the wood processing industry,containing 30% fibre and 70% water, given that dt=100-20=80° C. c_(s)=1500 j/kg°C.=1.5 kJ/kg°C., h₂ =2.676,000 J/kg°C.=2,676 kJ/kg°C. and h₁=83,900 J/kg°C.=83.9 kJ/kg°C., the energy consumption per kg will be:

    Q=1 0.3·1.5·80+0.7(2676-83.9)!=1,850 kJ/kg=1,850,000 kJ/ton=514 kWh/ton net.

Added to this will be the thermal losses from the process which may varyfrom 10% and upwards to several hundred percent. An energy consumptionof 3 times the net amount is not unusual.

If we now look at the energy required to transform water into mist, thestarting point must be the size of the mist droplets, which is between0.01 and 10 μm. If we elect to use (0.01+10)/2=5 μm as a kind ofaverage, then one liter of water may form a number of mist dropletsexpressed by: ##EQU1##

Of which:

    N=1000,000.sup.3 ·6/5.sup.3 μ=1.53·10.sup.13

The energy required to form these droplets can be assumed to equal theenergy of the total surface tension between water and air. The surfacetension of pure water is σ_(v) =71.4 dyn/cm=7140 dyn/m=7140 dyn·m/m²=7140·10⁻⁵ Nm/m² =7145·10⁻⁵ J/m². The total surface of the mist dropletsis:

    A.sub.tot =N·d.sup.2 π=1.53·10.sup.13 ·(5·10.sup.-6).sup.2 π=1200 m.sup.2

Total energy is:

    Q.sub.tot =A.sub.tot ·σ.sub.v =1200·7145·10.sup.-5 =85.74 J/kg=85,740 J/ton=85.74 kJ/ton.

This is in the order of magnitude of approx. 20,000 times less energythan by pure evaporation of water.

This phenomenon is currently being used for devices such as so-calledhumidifiers. Earlier models were based on pure evaporation, in that anelectric immersion heater was arranged in a container heating the wateruntil it boiled and evaporated. In present-day models the electricimmersion heater is replaced by a piezo-electric crystal oscillating atapprox. 20 kHz. When a drop of water hits the crystal, it cavitates intomillions of tiny water droplets together forming a mist which humidifiesthe room at room temperature.

The frequency of the sonic power required to achieve this is given by F.D. Smith as

    f= 1/(2π)!/ {3π(P.sub.o +2σ/r)}/o!.sup.0.5

where r is the radius of the droplets, μ is the specific heat of the gasin the droplets, P_(o) is the external hydrostatic pressure, o is thespecific weight (density) of the liquid and σ is the surface tensionbetween liquid and gas.

The equation relates to the cavitation in a liquid when it is exposed tosonic energy. Calculations for differently sized droplets show that thefrequency would reside in the area of 10⁴ to 10⁶ Hz.

The materials to be dried are not, however, a homogeneous mass. Theymight consist of a number of different materials as well as one or moredifferent types of liquids, although the predominant proportion wouldusually be water.

It is common knowledge that the liquids in such mixtures may exist as afree moveable liquid and as liquid bound by physical forces in thematerials, consisting predominantly of capillary forces. In order toconquer these forces in a normal drying process, the steam pressureneeds to conquer the capillary pressure which Increases in proportion tothe diminishing pore structure. In an ordinary drying process. This isachieved by drying the material at a temperature such that the steampressure conquers the capillary pressure. This will frequently requiretemperatures far above what is strictly necessary for evaporation.Another aspect affecting pure thermal drying is the fact that thethermodynamic values of the material change durlng the process. Theyworsen. This must In its turn be compensated by higher temperatures andlarger heating surfaces, which again leads to higher cost of equipmentand operations.

In order to achieve the objective of this invention--the drying as acombination of mist formation and evaporation, the elements supplyingthe sonic and thermal energy to the material need to be able to supplythese under such turbulent conditions that all parts of the material aretreated. By rendering the material into an extremely turbulent state, wewill not only have conditions with approximately constant thermodynamicdata, but we will also prevent the mass from getting entrenched in theprocess chamber and its constituent parts--which is a major problem inall other drying processes of the prior art.

In order to achieve the above conditions, a process is proposedconsisting of the following elements:

In a vertical or horizontal cylindrical container, an agitator isarranged driven by a rotating source of energy--an electric engine orcombustion engine. On the agitator are devised a series of pivotallysupported blades which, when the rotor rotates, are flung outwards bythe centrifugal force. The material is also flung outwardly by thecentrifugal force and the liquid is thus removed from the solidmaterial.

On the process chamber itself, an input opening is provided deviced forthe material which is to be dried, an output opening for the driedmaterial, and an opening for the evacuation of steam and mist.

In order to supply sonic energy to the turbulent mass, the followingprocedure may be applied:

On the inside of the process chamber, ribs are arranged runninglengthwise over the whole internal periphery of the chamber. The ribsare shaped so as to be approximately parallel with the blades where theypass the ribs. When the rotor rotates at n RPM having N blades per arm,each revolution will cause n·N pressure impulses between the rib and theblade, which are transferred to the turbulent mass in the chamber. Withe.g. a diameter of 2 m and 179 ribs at 35 mm intervals, 12 blades aroundthe rotor running at 2500 RPN, pressure waves will be generated fromeach blade at a frequency of 179·2500/60=7458 pulses per sec. Thesepressure waves will rip up the liquid and make a substantial portion ofit cavitate and form mist. In addition, heat will be generated from theblades due to internal friction in the mass and additionally by thesupply of heated air or another gas, e.g. CO₂ from a combustion engine.With these added gases, the mist will cause the generation of extremelylow partial pressure in the process chamber for the evaporating portionof the liquid. Experiments have shown that the partial pressure for thevapours will be so low as to give evaporation from 40°-75° C.

The gases, mist and steam leaving the process chamber through the outputopening from these may either condense or be led to air--depending onthe nature of the gases.

Another way of increasing the frequency and thus the mechanical effecton the liquids, is to design the blades as tuning forks, so as to be setin natural oscillation on passing the ribs, thus contributing to thesonic energy. As an alternative to the purely mechanical effect on thetuning forks, they may be electro-magnetically stimulated by a magneticoscillator arranged on the outside. This is devised so as to allow themagnetic field to go through the wall of the process chamber bydesigning the said chamber in a non-magnetic material (such as stainlesssteel).

The magnetic field will affect each of the blades designed as anoscillating element at the same frequency as the magnetic field. Afurther way of reinforcing the effect would be to arrange amagnetostrictive material on the blades, such as "Terfonol^(R) " madefrom the elements ferrum, terbium and dysprosium. Under the influence ofa magnetic field, the alloy will expand by approx. 1-2 o/oo. Thisexpansion may directly influence the blades and thus the material to betreated.

Yet another method of establishing a strong sonic field in the processchamber is to mount the ribs on piezoelectric ceramics ormagnetostrictive materials. When the correct impedance is establishedbetween the mass of the ribs, it will be possible by means of afrequency modulator on the outside to make the ribs oscillate at anychosen frequency within the practical range in question. In thefrequency range of 20,000 Hz no movement will be observable in the ribs,as they will be oscillating around their elasticity range.

Other ways of supplying vibrations may be envisaged, such as designingthe blades to give off a sound analogous to that of a whistle duringmovement in the mass. This can be achieved simply and practically bydesigning the backs of the blades so as to form extreme turbulencebehind the said blades. Apart from giving off vibrations, the turbulencewill also form a lagging vacuum which will render mist formation andevaporation more efficient at a low temperature.

Experiments have shown that it is possible to reduce the energyconsumption to less than half of the theoretical energy consumption fordrying e.g. wood fibre sludge.

The process is automatically governed by the following parameters:

Process temperature.

Lead on the engine.

Humidity of the dried material.

In other words, the output of dried material starts at a selectedoperating temperature and humidity. The resulting reduced load on theengine will cause input to start until the load again peaks. Duringinput, the temperature drops and output begins. In a correctly adjustedplant, this will be continuous, so that input will balance output, i.e.output and input will be continuous.

The process is shown in detail in the subsequent drawings, where:

FIG. 1 shows a schematic view of the process.

FIG. 2 shows a detail of the process chamber and the blade at the momentof passing.

FIG. 3 shows a detail of one of the blades designed as a tuning fork.

FIG. 4 shows the same fork prestressed with the magnetostrictivematerial "Terfonol^(R) " with an oscillating magnet generator.

FIG. 5 shows a detail of a rib attached to a piezoelectric ormagnetostrictive material.

FIG. 6 shows a side view of an embodiment example for an apparatus of450 kW_(o).

FIG. 7 shows a front view of the same embodiment example.

FIG. 8 shows the embodiment example seen from one end.

FIG. 1 shows a process chamber with rotor 2 and blades 3 driven by theengine 4. The mass is fed into the process chamber by the feed conveyor5 from the hopper 6. The feed conveyor is driven by the engine 7. Themass in the process chamber is whipped by the blades S and subjected tosonic energy or vibrations from the said blades and the ribs 8, whichare sufficiently closely spaced to each other to cause turbulence duringthe rotation of the blades. As additional energy may be supplied someform of heated gas from the combustion engine 9. The gases, mist andvapours leave the process chamber 1 via the output opening via a ventfan 11 and on to either open air or to a condenser. The dried materialis led through the output opening 12 via the rotating gate 13.

FIG. 2 shows, as mentioned, a detail of the process chamber and theblade at the moment of passing.

FIG. 3 shows a detail of one of the blades designed as a tuning fork.

FIG. 4 shows the same tuning fork prestressed with the magnetostrictivematerial Terfonol^(R) 14, with an oscillating magnet generator 15.

FIG. 5 shows a detail of a rib 4 attached to a piezoelectric ormagnetostrictive material 20 receiving oscillating energy via the cables21 and 22 from the generator 23.

FIG. 6, where the reference numbers have the same significance as inFIG. 1, also shows a screw conveyor 25 for the solids and the machineskid 16 on which the construction is resting. It further shows theblowoff pipe 17 for exhaust gases and mist being led to the open air.The control booth 18 for the process is situated at the end of theengine. The whole assembly is built into a 20 foot standard ISOcontainer 19.

FIG. 7 shows a top view of the same embodiment example, including theblowoff aperture 10 for the exhaust gases and mist, the vent fan 11 andthe feed conveyor 5 in the hopper 6. In the hopper are 4 transverse feedconveyors, whose only function is to feed the material on to feedconveyor 5.

FIG. 8 shows an end view from the left hand side of the embodiment shownin FIG. 6.

I claim:
 1. Apparatus for drying of organic or inorganic materials ormixtures thereof, the materials containing one or more liquids, wheredrying occurs by removing said liquid or liquids as atomized dropletsfrom the solids in the form of a mist, said apparatus comprising:aprocess chamber with an inlet opening for the material to be dried, saidprocess chamber having an inner wall; a rotor rotatably supported aboutan axis of the process chamber, the periphery of said rotor togetherwith the opposing inner wall of the process chamber defining an annulusin the process chamber, the rotor including on its periphery oscillatingrotor blades, and said inner wall of the process chamber havingsubstantially axially oriented ribs, said rotor blades on rotationcausing the material fed to the input opening to move radially outwardlyof the rotating rotor so that the liquid or liquids are removed from thesolid materials and said rotor blades being sufficiently closely spacedto the opposing ribs to impart turbulence during said rotation to theliquid outwardly removed from the solid materials and said oscillatingrotor blades imparting vibrations to said liquid so as to form a mist.2. Apparatus as claimed in claim 1, wherein each blade is pivotablysupported on an axis parallel to the axis of rotation of the rotor. 3.Apparatus as claimed in claim 1, wherein the rotor blades are designedwith highly turbulence-generating backs as viewed in the direction ofrotation.
 4. Apparatus as claimed in claim 1, wherein the rotor bladesare designed in the shape of tuning forks.
 5. Apparatus as claimed inclaim 1, wherein a magnetic oscillator is arranged outside the innerwall of the process chamber, and the rotor blades are associated with amagnetostrictive material.
 6. Apparatus as claimed in claim 1, whereinthe ribs are in the form of oscillating elements.
 7. Apparatus asclaimed in claim 6, wherein the ribs are mounted on piezoelectricceramic or magnetostrictive material.
 8. A method of drying organic orinorganic material or a mixture thereof, said material containing aliquid or liquids, which comprises introducing said material interiorlyof a rotating rotor carrying on its periphery oscillating rotor blades,said rotor rotating about the axis of a process chamber having an innersurface with substantially axially extending ribs, said rotating rotorcausing the material introduced interiorly of said rotor to be propelledradially outwardly of said axis of rotation and to expel liquid orliquids from said materials, and the oscillating rotor blades and theopposing ribs causing vibrational forces and turbulence to be inducedinto the outwardly flowing liquid so as to form a mist.
 9. A methodaccording to claim 8, which includes introducing heated gas into theprocess chamber to provide additional energy for drying of the material.10. A method according to claim 8, in which the material is wastematerial.
 11. A method according to claim 8, in which the ribs arecaused to oscillate by means of a frequency modulator applied from theoutside of the process chamber.